A wound rotor motor (WRM) can be controlled using a voltage source inverter (VSI) in combination with a voltage source converter (VSC) so that variable motor speed and torque are obtained. An advantage of a this type of system is that only a fraction of the power delivered by the motor to the shaft which it drives must be handled by the drive system, i.e. the VSI/VSC combination. This is a desirable feature since the cost of the drive system is proportional to its capacity.
However, in order to operate under VSI control a WRM must first be started and brought from inactivity to some speed so that the rotor voltage does not exceed the voltage ratings of the drive system. In the prior art, this has been achieved by reliance on expensive starting resistors and contactors. FIG. 1 illustrates such a conventional system where power is supplied from a utility at 10 through a stator isolation breaker 20 connected to the stator of a motor 30. Some power flows out of the rotor of motor 30 during starting when torque is required at the motor shaft and voltage is blocked by inverter isolation breaker 40 from reaching the motor drive elements formed by conventional VSI 50 and conventional VSC 60. Instead, this power is diverted through starting breaker 70 to resistance means 80, which can, for example, be a liquid rheostat or other similarly functioning device, and is dissipated there in the form of heat. When the speed of motor 30 has accelerated to the point that the rotor voltage no longer exceeds the ratings of the VSI 50 breaker 70 opens and inverter isolation breaker 40 closes and the motor continues operation under the control of the VSI 50. The point at which the transition from resistance means 80 to VSI 50 operation defines the minimum operating speed of the system under VSI 50 control.
The power requirement of the motor drive elements is governed by the power generated along the rotor circuit. This can be understood by using the following well-known equations:Pag=T×Ws Pm=T×Wm Prot=Pag−Pm where the rotor circuit power (Prot) equals the difference between the air gap power (Pag) and the mechanical power (Pm). The air gap power (Pag) is determined by the product of the shaft torque (T) times the power utility frequency along the mains (Ws), while the mechanical power (Pm) is determined by the product of the shaft torque (T) and the mechanical speed (Wm). From these equations, it becomes clear that slow motor speed and high torque, which can be the case at starting, can lead to high rotor circuit power. As a result, the conventional arrangement in FIG. 1 was developed to prevent the power capability of the VSI and VSC drive elements from being exceeded during starting when the speed of the motor is low and torque demands are high. However, this protection is achieved by means of inclusion in the starting circuit of expensive and bulky resistance and breaker devices.
What is needed is a more efficient and economical way to achieve this same goal by making the resistance and breaker components superfluous and eliminating them.